The present invention relates to an image forming apparatus incorporating therein a fixing unit capable of cleaning a fixing roller by means of an oil roller and a cleaning roller.
A fixing device incorporated in an electrophotographic image forming apparatus is configured to fix a toner image transferred to a sheet onto the sheet by fusing, mixing and pressure-contacting the toner image by means of a fixing roller incorporating therein a heat source, such as a halogen heater, and a pressurizing roller forming a fixing nip portion between the pressurizing roller and the fixing roller.
The fixing roller is brought into contact with the toner image transferred to the sheet and hence may be stained at its surface with toner. The toner stain thus adhering to the fixing roller causes succeeding sheets to be stained by adhering thereto.
In view of such an inconvenience, a technique has been disclosed for preventing accumulation of toner stain on the fixing roller by a post-heating process in which the temperature of the heating source is controlled to heat the fixing nip portion up to a temperature equal to and higher than the softening point of the toner after backward rotation of the fixing roller at the end of image formation (see Japanese Patent Laid-Open Publication No. 2006-276701).
Besides the technique described in Japanese Patent Laid-Open Publication No. 2006-276701, there is a fixing device including an oil roller and a cleaning roller for preventing toner stain from adhering to the surface of the fixing roller. This fixing device is configured such that the oil roller abuts against the fixing roller while the cleaning roller abuts against the oil roller.
This configuration can prevent the toner stain from adhering to the surface of the fixing roller by enhancing the toner release property of the fixing roller by the oil roller applying oil onto the surface of the fixing roller. However, the toner stain slightly adheres to the fixing roller even in a state of being applied with oil. Because the toner stain thus adhering to the fixing roller is transferred to the oil roller, such toner stain is recovered by the cleaning roller from the oil roller.
With this configuration, however, when the cleaning roller having recovered the toner stain is heated to an elevated temperature by transfer of heat from the fixing roller to the cleaning roller, the toner stain oozes out of the surface of the cleaning roller. The toner stain thus oozing out of the surface of the cleaning roller cools and is firmly fixed onto the oil roller after the end of printing. The toner stain thus firmly fixed is crushed at the beginning of next printing, which causes sheets to be stained.
In view of the problem described above, a feature of the present invention is to provide an image forming apparatus which is capable of preventing sheets from being stained by preventing the toner stain oozing out of the surface of the cleaning roller from being firmly fixed.